Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical layout of a conventional fuel cell 10 is shown in FIG. 1 which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane 11 is sandwiched between an anode 12 and a cathode 13. Typically, the anode 12 and the cathode 13 are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode 12 and cathode 13 are often bonded directly to the respective adjacent surfaces of the membrane 11. This combination is commonly referred to as the membrane-electrode assembly, or MEA.
Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate 14 and a cathode fluid flow field plate 15 which deliver fuel and oxidant respectively to the MEA. Intermediate backing layers 12a and 13a may also be employed between the anode fluid flow field plate 14 and the anode 12 and similarly between the cathode fluid flow field plate 15 and the cathode 13. The backing layers are of a porous nature and fabricated so as to ensure effective diffusion of gas to and from the anode and cathode surfaces as well as assisting in the management of water vapour and liquid water. Throughout the present specification, references to the electrodes (anode and/or cathode) are intended to include electrodes with or without such a backing layer.
The fluid flow field plates 14, 15 are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode 12 or cathode electrode 13. At the same time, the fluid flow field plates must facilitate the delivery and/or exhaust of fluid fuel, oxidant and/or reaction product to or from the porous electrodes.
This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels 16 in the surface presented to the porous electrodes 12, 13. Hydrogen and/or other fluid fuels or fuel mixes are delivered to the anode channels. Oxidant is delivered to the cathode channels, and reactant product water vapour is extracted from the cathode channels.
Throughout this specification, the expression “channel” will be used to indicate any suitable conduit for delivery of fluid fuel or oxidant to the MEA and/or for the exhaust of unused fuel or oxidant together with any purge or reactant products from the MEA.
With reference also to FIG. 2(a), one conventional configuration of fluid flow channel 16a in the cathode fluid flow field plate 15 for delivery of oxidant to, and exhaust of water vapour from, the MEA is an open-ended channel having an inlet 21 and an outlet 22. This allows a continuous through-purge of gas to provide the requisite exhaust purge.
With reference also to FIG. 2(b), one conventional configuration of fluid flow channel 16b in the anode fluid flow field plate 14 for delivery of hydrogen fuel to the MEA is a “dead-ended” channel arrangement 16b, typically in a comb-like structure. Such a dead-ended channel 16b has an inlet 24, but no outlet, the hydrogen fuel being consumed as it enters the MEA from the channels 16b. As shown, two interdigitated comb structures may be used, with two inlets 24.
For simplicity, the channels 16b are shown in this diagram simply as single lines although it will be understood that they have finite width. An outline of an underlying open-ended cathode channel 16a is shown in dashed outline. The depiction of the channels 16 in the drawings is highly simplified for clarity; the channel widths and separations may both be of the order of a millimetre or so.
The dead-ended channel arrangement for the anode channels 16b suffers from at least one significant disadvantage. Although the reactant product, typically water vapour, is primarily produced on the cathode side of the MEA, and can be exhausted from the open-ended channel outlet 22, some water is typically transported back to the anode side of the MEA by diffusion. Unless managed, this water can accumulate locally and impede the access of hydrogen to the catalytically active sites for electrochemical reaction, effectively deactivating the portions of the electrode from which the hydrogen is blocked. This is sometimes referred to as ‘flooding’ of the anode and results in gradual but persistent performance decline in the fuel cell. A lower power output capability at any given operating voltage is the result.
In the prior art, one solution to this problem is to also use an open-ended channel 16a as the anode channel, allowing a continuous or intermittent purge of excess hydrogen to exit the fuel cell, carrying water with it to remove the water from the ‘water masked’ surfaces, thereby re-admitting hydrogen to the previously blocked sites.
It will be recognised that this is wasteful of hydrogen fuel which is either lost as an exhaust gas, or else it must be dehumidified and/or reconditioned so that it can be recycled to the fuel inlet. This can contribute substantially to overall system inefficiencies or complexity of fuel delivery equipment and therefore large volumes of unused purge hydrogen are undesirable.